Asylum seekers should not be expected or required to lie about their political beliefs, the supreme court has ruled in a decision likely to make it more difficult to deport asylum seekers to Zimbabwe.

In a unanimous judgment the court dismissed an appeal by the home secretary, citing a campaign of persecution by "undisciplined militias" against those who do not profess active support for president Mugabe's Zanu-PF party.

The complex ruling by seven justices says that the right not to hold political opinions should be protected by the 1951 Refugee Convention in the same way as the right to hold religious views or other beliefs. In a truly democratic society, Lord Dyson explained, individuals should be free not to hold opinions.

The Home Office had attempted to deport several asylum seekers, identified by the court only as RT and KM, to Zimbabwe on the grounds that they would not be in danger if returned. They had not been members of the opposition Movement for Democratic Change or active opponents of the government.

The Immigration and Asylum Tribiunal's country guidance for Zimbabwe, the court noted, acknowledges there is a "campaign of persecution perpetrated by undisciplined militias who have delivered a quite astonishingly brutal wave of violence to whole communities thought to bear responsibility for the 'wrong' outcome of the March 2008 election".

Anyone who cannot demonstrate positive support for Zanu-PF or alignment with the regime is at risk. "The means used to establish loyalty include requiring the production of a Zanu-PF card or the singing of the latest Zanu-PF campaign songs. Inability to do these is taken as evidence of disloyalty and therefore support for the opposition," the court accepted.

In its judgment, the supreme court said: "There are no hierarchies of protection amongst the Refugee Convention reasons for persecution. Thus the Convention [to which the UK is signatory] affords no less protection to the right to express political opinion openly than it does to the right to live openly as a homosexual.

"... The right to freedom of thought, opinion and expression protects non-believers as well as believers and extends to the freedom not to hold and not to have to express opinions.

"There is no basis in principle for treating the right to hold and not to hold political beliefs differently from religious ones. There can also be no distinction between a person who is a committed political neutral and one who has given no thought to political matters.

"... Persecution on the grounds of imputed opinion will occur [in Zimbabwe] if a declared political neutral is treated by the regime as a supporter of its opponents and persecuted on that account."

Expanding on the principle, Lord Dyson noted: "It can therefore be seen that under both international and European human rights law, the right to freedom of thought, opinion and expression protects non-believers as well as believers and extends to the freedom not to hold and not to express opinions...

"It is the badge of a truly democratic society that individuals should be free not to hold opinions. They should not be required to hold any particular religious or political beliefs. This is as important as the freedom to hold and (within certain defined limits) to express such beliefs as they do hold.

"One of the hallmarks of totalitarian regimes is their insistence on controlling people's thoughts as well as their behaviour.George Orwell captured the point brilliantly by his creation of the sinister 'Thought Police' in his novel 1984. The idea 'if you are not with us, you are against us' pervades the thinking of dictators."

Responding to the decision, a Home Office spokesperson said: "We are disappointed at the outcome of this appeal. We are still considering the full implications of the judgment but we will continue to consider each asylum claim on its own merits.

"We remain committed to granting asylum to those who need it and ensuring that those who do not return to their own country."